HPK1-Mediated Lymphocyte Signal Transduction Mechanisms. The JNK kinase cascade plays a pivotal role in T-cell activation, differentiation and apoptosis. Hematopoietic progenitor kinase 1 (HPK1), a STE20-1ike kinase, is a hematopoietic-specific activator of the JNK signaling pathway. Our working hypothesis about the HPK1 signaling pathway is as follows: Receptors -> adaptors, kinases -> HPK1 (also called MAP4K1) -> MAP3Ks (MEKK1, TAK1, MLK3) -> MAP2Ks (MKK4, MKKT) !->JNK -> cell activation or apoptosis. This application plans to study the function of HPK1 in T-cell activation and T-cell mediated immune responses. In this application, we will test the hypothesis that HPK1 negatively regulates T-cell activation by inhibiting ERK and/or activating JNK. The approach will be: (i) to understand the underlying signal transduction mechanisms of the HPK1 pathway in the negative regulation of T-cell activation, and (ii) to study the in vivo functions of HPK1 in Th1/Th2 differentiation, immune responses, autoimmunity, and professional antigen-presenting cell (APC) function. Ultimately, we plan to dissect various HPK1-mediated signaling pathways in immune responses, leukemia/lymphoma, and immunological diseases. Our future understanding of host factors involved in the HPK1-mediated lymphocyte signaling pathways will provide information fundamental to the discovery, design, and evaluation of effective intracellular therapeutic agents for leukemia/lymphoma and immunological disorders such as autoimmunity. The specific aims are: AIM 1. Study the roles of HPK1 in T-cell activation and differentiation using HPK1 knockout mice. AIM 2. Study in vivo functions of HPK1 in T-cell mediated immune responses, autoimmunity, and antigen-presenting cell (APC) function using HPK1 knockout mice.